An antenna circuit in a mobile terminal functions to transmit a predetermined radio signal through an antenna or receive the signal. In order for an antenna to have an optimal transmission/reception radiation performance, impedances are correctly matched. Therefore, the antenna circuit includes a capacitor and an inductor, where capacitor and inductor values are controlled to match the impedances of the antenna in the optimal state.
In general, the impedance matching is to match impedances of an antenna while a mobile terminal is positioned at a free space. Meanwhile, a mobile terminal is characteristically used in such a manner where a body of the mobile terminal is held by a hand and a speaker is touched to an ear, or the body of the mobile terminal is put into a pocket or a bag using an earphone.
According as a user utilizes the mobile terminal by holding a body of the mobile terminal, touches the body to an ear, or puts the body into a pocket or a bag, an impedance matching condition of an antenna becomes variable, whereby the transmission/reception radiation performance of the antenna whose impedance has been matched in a free space inevitably decreases.
As a result, a mobile terminal adopts an adaptive tuning antenna circuit having an antenna that has an optimal transmission/reception radiation performance by automatically controlling impedance of the antenna, in a case the impedance matching condition of the antenna is changed. It is imperative that a changed impedance state of an antenna be detected in order for the adaptive tuning antenna circuit to maintain in an optimal state. To this end, the adaptive tuning antenna circuit is coupled with a coupler and detects a reflected power and a forward power outputted by the coupler. A tuner is adjusted to control the impedance of the antenna based on the detected reflected power and the forward power.
However, in order to control the impedance of the antenna by detecting the reflected power and the forward power, it is essential that two power detectors be used to detect the reflected power and the forward power. The impedance control is to actually measure the reflected power reflected from an antenna that is resultant from an impedance mismatching of the antenna. Therefore, there occurs a problem in which an output of a signal emitted from an antenna is actually changed in the course of optimally matching the impedance of the antenna.